As described in, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-31967, a known deceleration controller for a vehicle, such as an automobile, performs, as a warning to a driver, warning braking for decelerating the vehicle by means of braking when deceleration is needed from the viewpoint of a risk of colliding with a front obstacle and securing of safety. According to such a deceleration controller, in a situation where deceleration is needed from the viewpoint of a risk of colliding with a front obstacle and securing of safety, a warning which indicates the necessity of deceleration can be issued to a driver by means of deceleration of the vehicle.
In general, the warning braking is performed in a situation where the driver does not carry out brake operation even though the vehicle is likely to collide with a front obstacle. Such warning braking is performed by decelerating the vehicle by means of braking in order to issue a warning to the driver. Therefore, when deceleration of the vehicle has already been started and the degree of the deceleration is large, the warning braking is not effective. Further, when the warning braking is performed to decelerate the vehicle which is already in a decelerated state, a change in the deceleration of the vehicle is small, and a crew of the vehicle cannot recognize the warning braking in some cases.
In particular, in some cases, the vehicle is decelerated in response to a deceleration request associated with travel control of the vehicle, such as cruise control, and the difference between a deceleration of the vehicle achieved by the warning braking and the deceleration achieved in response to the deceleration request is small, even thought the former deceleration is greater than the latter deceleration. In such a case, even through the crew of the vehicle can recognize a change in the deceleration of the vehicle due to execution of the warning braking, the crew recognizes it as a mere change in deceleration during travel of the vehicle, not as a warning. Therefore, the warning braking causes the crew of the vehicle to feel an unnatural sensation.